Spark-gap device

ABSTRACT

A miniature three-element spark-gap device comprises a ceramic enclosure supporting aligned end electrodes extending therewithin, and a solid disc center electrode establishing first and second gaps with the end electrodes. The solid disc center electrode is apertured so that an arc discharge across one gap will illuminate the other, producing ionization and causing immediate breakdown of the other gap.

United states Patent [191 Kawiecki 1 May 14,1974

[ 1 SPARK-GAP DEVICE [75] Inventor:

Calif.

[73] Assignee: Joslyn Mfg. and Supply Co.,

- Chicago, 111.

[22] Filed: Dec. 20, 1972 I [21] Appl. No.1 316,736

521 u.s.c| ..313/32s,313/3o9,31s/36 51 Int.Cl .1101 l7/00,H0|j2l/0()' [58] Field of Search 315/36; 313/325, 309

[56] References Cited UNITED STATES PATENTS 2,290,526 7/1942 Berkey at al 313 325 x 2,365,595 12/1944 Ronian 313/325 x Chester .1. Kawieckl, Santa Barbara, I

l/1964 Gardiner et a1 313/309 X 6/1971 Kawiecki 313/325 Primary Examiner-James W. Lawrence Assistant Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or FirmKlarquist, Sparkman, Campbell, Leigh. Hall & Whinston I l 57] ABSTRACT A miniature three-element spark-gap device comprises a ceramic enclosure supporting aligned end electrodes extending therewithin, and a solid disc center electrode establishing first and second gaps with the end electrodes. The solid disc center electrode is apertured so that an arc discharge across one gap will illuminate the other, producing ionization and causing immediate breakdown of the other gap.

12 Claims, 5 Drawing Figures PATENTEDHAY 14 1974 8 05 DUEJEJEI DEEDS 1 SPARK-G'AFDEVICE BACKGROUND OF TH-EINVENTION Spark-gap devices are frequently. employed .as protectors acrossvoltage supply lines, telephonelines, and the like, for: protecting. equipment fromtransient. surges. For. example, such aspark+gapdevicemaybe; employed as a lightningarrester'prov'iding a breakdown path to ground when atlightning surgeoccurs, whereby such surge does not reach and t destroy. the apparatus connected tothe-line.

A suitable spark-gap configuration for protectingequipment connectedto a line'paincomprisesa;threet terminal arrangementby means of which a high .voltage transientoneither or both lines-may bewshunted ,to. ground. A pair oflspark gaps withceach oneudisposed. between.-a -line and groundtmay beemployed; but in 1 such'case' the .two devices may notdi'schargesimulta neously; often leaving .an unbalanced high. voltage cou.-

' number ofsurges during its operating lifetime than the last mentioned two-gapdevice. The present device,

pled to the device to be prote.cted..0rie popularconfig-j uration comprises a three electrode device :including. end electrodes in= juxtaposed relation, and a cylindrical.

ground electrodefsurrounding the gap between the. end i.

electrodes. W- hilearcing desirably takesplace between each end .electrodeeand :the; grounded *cyli'nder-atsubt stantially the-same: time, a discharge.:.may;.occur.. be.-- tween the end. electrodes. without: involving the grounded electrode. A highly advantageous three-element. construction which may be utilizedto provide the desiredegrounding; protection :isdis'cloSedand claimedin my U.S.- Pat; No.

also being-shorter, provides faster communication of ionization between the'two gaps. Since the device is smaller in overall size, it is moretcompatible with the modern day miniature equipment andis also easier and more economical to produce than other devices of this type. v

It isaccordingly an object of the present invention to provide an improved spark-gap device which effectively and substantially simultaneously grounds a pair of-lines intheevent ofa transient surge.

lt isa'further object of the present invention to provide an improved spark-gap device of the three. element type for shunting high voltage surges to ground,

wherein such device is sturdy in construction and capableof-withstanding..heavier surges and' a greater number of surges duringits operating lifetime;

Itiis a .furtherobject of the present invention to providean. improvedthree-element, two-gap device providing. faster communication of ionization between its two gaps.

It is another. object of 'the present invention to provide anzimproved spark-gap device which is smaller,

more compact; less costly, and more compatible with' miniatureequip'ment than has been thecase in prior 3,535,582 entitled; Unitary; Series: Spark-gap. with" Aligned. Apertures. issuedzOCtiZO, I970, andassigned: to the .assigneeof-"the: presenta invention. lithe: construction: t according; to :the. previouswpatentpax pair of spark-gap devices are: aligned. in end toaend-lzrelation I with a passage communicatingtbetween-ithe.gapsillumi-, natingx one gap when -the oth'er' 'discharges, whereby substantially immediate ionizationtakes place bringing. about a discharge acrossboth gaps. The .gapaccording: to the present invention is an improved construction of this general type;

ISUMMAIQY OF THE-INVENTION In accordance withzthepresent invention, a.spark-.

' gap device includes first and second aligned conductive to thezpresentinvention, connected in circuit with a desired to shunt'discharges'to ground; and thusthe cen-.

ter electrodeis' generally grounded;

The device according to the. present invention=havingw the solid disc .c'enter electrode.- has the advantage. of

sturdy but'comp'act constructionwherebythe device is able -.to withstandlh'eavier surges and conduct: agreater devices.

Itis a further object of the present invention to provide-animproved miniature spark-gap device which is extremely small in size butoffers' enhanced grounding protection byshunting discharges from line'to ground forprotecting equipment connected therewith.

Thefsubject matter which-I regard as my invention is particularly pointedout and distinctly claimed inthe concluding portion of this specification. The invention,

however; both as to organizationand method of"opera-. tion, together withv further advantages. and objects thereof; may best be-sunderstood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters'refer tolike elements.

v DRAWINGS FIG..1 is a side view of a spark-gap device according load,

FIG. 2 is an enlarged longitudinal cross-section of the spark-gap'device according to the present invention;

FIG. 3 is an'e'nd view of such spark-gap device; FIG. 4 is a first transverse cross-sectional view of the spark-gap device accordingto; the .present invention taken-at 4-4-in FIG 2; and

FIG. 5 vis a second transverse cross-sectional view of the'spark-gap device according to the'present invention taken at 55 of FIG 2. Y

DETAILED DESCRIPTION Referring to the drawings and-particularly to FIG. 1, a spark-gap'device accordingto thepresent invention includes hollow enclosure means comprising cylindrical alignedspacer tubes l0'and 12 preferably taking the form of glass or ceramic tubes. The device is quite small'and'in a particular instancehad a total length of approximately 0.46 inch, with each of the cylindrical spacertubes being approximately 5/ I 6 inch in diameter and? approximately 3/16 inch in length. The spacer tubesare more clearly shown in FIG. 2 wherein the are joined to'the spacer tubes in sealing relationship.

Second closed ends, 22 and 24, of the electrodes are positioned within the respective spacer tubes and extend laterally nearly across the spacer tubes to provide fairly large, lateral, arc-supporting surfaces 26 and 28 disposed in opposed or facing relation. The surfaces 26 and 28 are suitably substantially perpendicular to the axial or longitudinal center line of the device and are preferably substantially planar over the major portion thereof. Each of these surfaces is desirably provided with a plurality of cavities 30 containing a low work function material such as potassium chloride for supplying a source of electrons and inhibiting electrode erosion due to cathode sputtering. The hollow electrodes l4 and 16, suitably formed of Kovar, may be generally cylindrical between their aforementioned first and second ends. Cylindrical outer walls 32 and 34 v and 12 and shield the spacer tube inner walls from electrode sputtering. The ceramic-spacer tubes and 12 are preferably formed of alumina.

t A third conductive electrode 36 is disposed between thefirst and second electrodes 14 and 16 where-it is supported by spacer tubes 10 and 12. The third electrode comprises a solid disc including a first portion 38 having a first thickness in a direction axial of the device, and a peripheral portion 40 of reduced thickness disposed between and joined in sealing relation to second ends of the aligned spacer tubes 10 and 12. The electrode 36. is further provided with an axial flange 66 at the outer edge of peripheral portion 40, such flange extending longitudinally over spacer tubes 10 and 12 for a short distance. The flange provides a good exterior contact and is also useful in centering tubes 10 and 12 during manufacturing of the device.

' The portion 38 having the said first thickness extends laterally across most of the cross-sectional area inside the spacer tubes, defining rounded outer shoulders 42 and 44' where the said first portion is joined to the peripheral portion 40 of reduced thickness. Shoulders 42 and 44 are adjacent the spacer tubes 10 and 12 and protect the joint formed between portion 40 and the spacer tubes-from the electrode arcing within the device. The electrode portion 38 thus protrudes longitudinally within the second ends of spacer tubes 10 and 12, presenting substantially parallel lateral faces 46 and 48 which are in exposed juxtaposition with the exposed lateral arc-supporting surfaces 26 and 28 of electrodes l4 and 16, forming gaps 47 and 49. The surfaces 26, 46, 2 8, and 48 are substantially parallel to one another andthe gap distances for gaps 47 and 49 are substantially equal.

The central portion 38 of electrode 36 is provided with an axial bore 50, desirably of constant diameter, forming a passage between the aforementioned gaps whereby an'arc discharge across one of said gaps will immediately illuminate the other and cause substantially simultaneous breakdown and discharge across the second gap. The diameter of this axial bore is desirably between approximately l /3 and 3/4 the diameter of the arc-supporting surfacces 26 and 28, i.e., the bore is not ,so large that spark-gaps 47 and 49 no longer exist in longitudinal direction between substantially lateral and parallel electrode surfaces of substantial size. Then, principal discharges occur between surfaces 26 and 46, and between surfaces 28 and 48, with electrode 36 grounded. Electrode 36 intercepts or becomes involved in longitudinal discharges because it is interposed as a partial barrier between arc-supporting surfaces 26 and 28.

Arc-supporting surfaces 26 and 46 may be thought of, in a narrow sense, as comprising the end parts of electrodes 14 and 16 for which it can be said the sum of distances a and b is less'than c, along substantially any longitudinal line intersecting the electrode ends. Thus arcing will preferentially take place in gaps 47 and 49 rather than along path c. Even though the faces 26 and 28 or'peripheral portions thereof can be somewhat rounded, severly taperedend electrodes are not as desirable since the center electrode could then lie outside a principal longitudinal arcing path between the end electrodes. Theconstruction according to the present invention is unlike prior constructions employing a cylindrical center electrode radially spaced from the arcing path between principal end electrodes. Thus, according to the present invention the tendency will be for arcing to take place to the center electrode 36 for grounding the transient, with such electrode 36 further forming a barrier, subsequently acting to break up the arcing path and extinguish the discharge.

In assembling the spark-gap device according to the present invention, the ends of the ceramic spacer tubes are suitably metallized as indicated at and 72 with a high temperature metal or alloy, i.e., molybdenum plus manganese. Brazing washers 68 and 74, which may be formed of an alloy of silver and copper, may be positioned on the metallized ends, and electrodes 14 and 16 are inserted into the first open ends of tubes 10 and 12. The tubes are aligned with electrode 36 interposed therebetween, the outer axial flange 66 assisting in the correct alignment of the structure. The assembly as described, with brazing washers in place, is suitably raised in temperature to braze the assembly with the interior thereof being evacuated and suitably provided with an internal gaseous environment at less than atmospheric pressure facilitating breakdown at fairly low transient voltages. A radioactive material may be provided. The

completed structure provides hermetically sealed chambers for the respective gaps 47 and 49, said chambers being joined by the bore 50 in electrode 36. Although the disc-shaped electrode 36 may be formed of Kova'r, i.e., the material'from which electrodes 14 and 16 are formed, the electrode 36 is more suitably formed of copper and fabricated by means of a coining operation.

A spark-gap according to the present invention is connected to a line to be protected as illustrated in F IG 1 wherein electrodes 14 and 16 are connected respectively to conductors S4 and 56 extending from a main line or source of power at terminals 58 and 60 to a load or other utilization device 62. A ground connection 64 is also provided which is connected to outer flange 66 of center electrode 36.

When a predetermined voltage level is reached, e.g., as a result of a high voltage transient on the line, gaps 47 and 49 break down into an arc discharge, thereby shorting out the high voltage transient to ground and sage, but also the initiation of an arc discharge across one gap provides radiation illuminating the region of the other gap. This illumination produces photoelectrons atan. electrode surface of the opposite gap,

which in turn causes ionization of the gas in the region of such gap. For example, assuming gap 47 is the first to break down, the arc discharge at gap 47 will illuminate surface 28 of electrode 16. through bore 50. The photons reaching surface 28 will produce photoelectrons which will be emitted from surface 28 then, which in turn will cause ionization of gap 49. As a result, gap 49 breaks down substantially immediately into an arc discharge. The time lag during which the foregoing events take place is such that both gaps breakdown at substantially the sametime. Theterms illumination and radiation employed above are meant to comprehend ultraviolet and/or visible radiation.

Thus both gaps break down to ground at the same time, usually shorting the same voltage to ground in a balanced manner andproviding optimum protection to equipment. Even though a line-to-line dischage may be present or may start first, each line is effectively grounded because of the intercepting interposition of electrode 36. Therefore the cooperation achieved with electrode 36 is such that arcs are struck to the center electrode in a simultaneous balanced manner.

A further advantage of the present invention, as indicated above, is action 0felectrode36 in breaking up or extinguishing the arc discharge when an'over-voltage condition is removed. The arcdischarge more easily extinguishes thanhas frequently been the case with prior art three-electrode devices, after the high voltage transient has subsided. The electrode. 36 acts as a barrier for quenching arcs in the extended arc path therethrough.

There is provided according to the present invention a small size spark-gap device-which is suitably constructed to break down and providearc discharges to ground at relatively lowtransient voltage values. Moreover, discharge to ground occurs between each electrode and the center or grounded electrode, substantially simultaneously, enhancing the protection -afforded. At the conclusion of the high voltage transient,

the spark gap device according to the present invention more quickly extinguishes the arc with the center of grounded electrode acting as a barrier for helping to quench the discharge. The device is structurally strong, enabling the same to withstand heavier sruges and a greater number of surges during its-operating lifetime. Furthermore, the three-electrode device according to the present invention is extremely compact and compatible with miniature equipment, being nearly'as small as prior two-electrode devices, while also being simple and economical to fabricate. The shortness of the device, and particularly, the shortness of electrode 36, brings about faster communication of ionization be tween gaps 47 and 49. i

The device according to the present invention is susceptible to variation without departing from the inventive concept. For example, the center electrode 36 is illustrated as having an axially wider central portion 38, which is of advantage in providing shoulders 42 and 44 for protecting the joint between tubes 10 and 12 and electrode 36. Moreover, the added thickness aides in positioning the ceramic tubes and in decreasing the gap distances for gaps 47 and 49 while lengthening the gap distance directly between surfaces 26 and 28. However, in many instances the disc electrode 36 is suitably entirely flat, i.e., with central portion 38 no wider than peripheral portion 40.

While I have shown and described a preferred embodiment of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

Iclaim:

l. A spark-gap device suitable for providing transient protection to a line above a predetermined voltage level, said device comprising:

a first and second aligned conductive end electrode means, and hollow enclosure means therebetween, said electrode means extending into said hollow enclosure means in opposed relation,

andv a third-electrode means disposed between said first and second electrode means and supported by said enclosure means, said third electrode means comprising a solid conductive disc provided with a direct passage therethrough providing a line-ofsight path between said first and second electrode means, said disc having lateral faces exposed in facing, longitudinal juxtaposition with exposed arcsupporting surfaces of said first and second electrode means to provide spark-gaps between said first and second electrode means and said disc, said disc forming an effective barrier between exposed arc-supporting surfaces of said first and second electrode means except for said passage so that said disc becomes involved in arc discharges which occur in said device, I said end electrode means, said disc and said hollow enclosure means being joined to provide closed chambers between said end electrode means and said-disc joined by the passage in said disc.

2. A spark-gap device suitable for providing transient protection to a line above a predetermined voltage level, said device comprising:

hollow cylindrical insulating spacer means open at opposite ends thereof, a first conductive electrode which is substantially cup-shaped in overall configuration having a radial flange at its first end extending over and joined to surface within said spacer means oriented longitudinally inwardly thereof toward the second end of the first electrode,

and a conductive solid disc peripherally supported by said spacer means and positioned between second ends of said conductive electrodes, said disc having a central axial bore providing a passage exposing said arc-supporting end surfaces to one another, said disc having its opposite lateral faces exposed in facing juxtaposition to substantially parallel exposed arc-supporting end surfaces of said first and second conductive electrodes to provide longitudinal gaps between the lateral faces of said disc and said first and second conductive electrodes.

3. A spark-gap device suitable for providing transient protection to a line above a predetermined voltage level, said device comprising:

a first cylindrical insulating spacer provided at first end thereof with a first conductive electrode which is substantially cup-shaped in overall configuration having a radial flange at its first end extendingover and joined to the first end of the first insulating spacer, the second end of said conductive electrode being closed and disposed within said spacer to provide an exposed lateral arc-supporting end surface within said spacer oriented longitudinally inwardly thereof, 4 second cylindrical insulating spacer axially aligned with the first spacer and provided at a first end thereof with a second conductive electrode which is substantially cup-shaped in overall configuration having a radial flange at its first end extending over and joined to the first end of the second insulating spacer, the second end of said second conductive electrode being closed and disposed within said second spacer to provide an exposed lateral arcsupporting end surface within saidspacer oriented longitudinally inwardly thereof toward the second end of the first electrode,

and a conductive solid disc peripherally joined to said first and second spacers at second ends thereof, said disc having a central axial bore providing a passage exposing said arc-supporting end surfaces to one another, said disc having opposite lateral faces exposed in facing juxtaposition to substantially parallel exposed arc-supporting end surfaces of said first and second conductive electrodes to provide longitudinal gaps between the lateral faces of said disc and said first and second conductive electrodes,

said electrodes, said disc and said spacers being peripherally joined to provide hermetically sealed chambers between said electrodes and said disc joined by the bore in said disc.

4. The device according to claim 3 wherein said disc has a first thickness extending within said spacers providing said opposite faces in juxtaposition with said exposed arc-supporting end surfaces, said disc having a peripheral portion of reduced thickness between and joined to said spacers, with said disc just inwardly from said peripheral portion defining a shoulder adjacent joints formed between said peripheral portion and said spacers.

5. The device according to claim 4 wherein said disc is further provided with an axial flange at the outer edge of said peripheral portion of reduced thickness, said axial flange extending adjacent and over a portion of at least one of saidcylindrical insulating spacers.

6. The device according to claim 3 wherein said central axial bore in said disc has a diameter between approximately on'e-third and three-fourths of the diameters of said end surfaces of said conductive electrodes.

ond ends of said first and second conductive electrodes providing said arc-supporting end surfaces are substantially planar over the major portion thereof.

8. The device according to claim 3 wherein the second ends of said first and second conductive electrodes providing said arc-supporting end surfaces are provided with a plurality of small cavities containing low work function material.

9. The device according to claim 3 wherein the sum of the longitudinal distances between said arcsupporting end surfaces and the lateral faces of said disc along substantially any longitudinal line intersecting said arc-supporting end surfaces is less than the closest longitudinal distance between said arc supporting end surfaces.

10. The device according to claim 9 wherein the diameter of the bore in said disc is between approximately one-third and three-fourths of the diameter of the said arc-supporting end surfaces.

11. The device according to claim 3 wherein the portions-of said electrodes between the ends thereof are substantially cylindrical and closely adjacent the cylindrical interior walls of said spacers.

12. The device according to claim 3 wherein said spacers are formed of ceramic material.

UNITED s'm'ms WWW omm'c' CERTHMCAEE @l CCMWCTWN Patent No. 3,811,064 Dated May 14, 1974 Inventor(s) Chester J, Kawiecki It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 54, "pate" should be pa.th-. I Column 4, line 1, "surfacces" should be --surfaoes-.

Column 5, line 54, "of" should be --or--.

Column 5, line 57, "sruges" should be -surges--. Column 6, li ne S, "aides" should be -aids-,

Column 7, line 18, after "at" insert -a-. I I

Column 8, line 22, "diameters" should be -diameter Signed and sealed this 5th day of November 1974,

(SEAL) Attest:

McCCY M, GIBSON; JR. c, MARSHALL DANN Attesting Officer Commissioner of Patents FORM Po-wso' (10-69) USCOMM-DC 60376-P69 fl U,S. GOVERNMENT PRINTING OFFICE: I969 0-366-334 UNITED STATES PATENT oTTmTz' CERTHICA'TE @F CQRRECTTOFN Patent No. 3,811,064 Dated May 14, 1974 Inventor(s) Chester J. Kawiecki It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 54, "pate" should be -path.

Column 4, line 1, "surfacces" should be surfaces.

Column 5, line 54, "of" should be -or- Column 5, line 57, "sruges" should be -surges-.

Column 6, line 5, "aides" should be -aids,

Column 7, line 18, after "at" insert a. I

Column 8, line 22, "diameters" should be diameter Signed and sealed this 5th day of November 1974.

(SEAL) Attest:

McCOY M, GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-IOSO (IO-69) USCOMM DC o376 P6g w u.s, sovsnuusm- PRINTING omcs: was o-aee-au 

1. A spark-gap device suitable for providing transient protection to a line above a predetermined voltage level, said device comprising: a first and second aligned conductive end electrode means, and hollow enclosure means therebetween, said electrode means extending into said hollow enclosure means in opposed relation, and a third electrode means disposed between said first and second electrode means and supported by said enclosure means, said third electrode means comprising a solid conductive disc provided with a direct passage therethrough providing a lineof-sight path between said first and second electrode means, said disc having lateral faces exposed in facing, longitudinal juxtaposition with exposed arc-supporting surfaces of said first and second electrode means to provide spark-gaps between said first and second electrode means and said disc, said disc forming an effective barrier between exposed arc-supporting surfaces of said first and second electrode means except for said passage so that said disc becomes invoLved in arc discharges which occur in said device, said end electrode means, said disc and said hollow enclosure means being joined to provide closed chambers between said end electrode means and said disc joined by the passage in said disc.
 2. A spark-gap device suitable for providing transient protection to a line above a predetermined voltage level, said device comprising: hollow cylindrical insulating spacer means open at opposite ends thereof, a first conductive electrode which is substantially cup-shaped in overall configuration having a radial flange at its first end extending over and joined to a first end of said insulating spacer means, the second end of said conductive electrode being closed and disposed within said spacer means to provide an exposed lateral arc-supporting end surface within said spacer means oriented longitudinally inwardly thereof, a second conductive electrode which is substantially cup-shaped in overall configuration having a radial flange at its first end extending over and joined to the second end of said insulating spacer means, the second end of said second conductive electrode being closed and disposed within said spacer means to provide an exposed lateral arc-supporting end surface within said spacer means oriented longitudinally inwardly thereof toward the second end of the first electrode, and a conductive solid disc peripherally supported by said spacer means and positioned between second ends of said conductive electrodes, said disc having a central axial bore providing a passage exposing said arc-supporting end surfaces to one another, said disc having its opposite lateral faces exposed in facing juxtaposition to substantially parallel exposed arc-supporting end surfaces of said first and second conductive electrodes to provide longitudinal gaps between the lateral faces of said disc and said first and second conductive electrodes.
 3. A spark-gap device suitable for providing transient protection to a line above a predetermined voltage level, said device comprising: a first cylindrical insulating spacer provided at first end thereof with a first conductive electrode which is substantially cup-shaped in overall configuration having a radial flange at its first end extending over and joined to the first end of the first insulating spacer, the second end of said conductive electrode being closed and disposed within said spacer to provide an exposed lateral arc-supporting end surface within said spacer oriented longitudinally inwardly thereof, a second cylindrical insulating spacer axially aligned with the first spacer and provided at a first end thereof with a second conductive electrode which is substantially cup-shaped in overall configuration having a radial flange at its first end extending over and joined to the first end of the second insulating spacer, the second end of said second conductive electrode being closed and disposed within said second spacer to provide an exposed lateral arc-supporting end surface within said spacer oriented longitudinally inwardly thereof toward the second end of the first electrode, and a conductive solid disc peripherally joined to said first and second spacers at second ends thereof, said disc having a central axial bore providing a passage exposing said arc-supporting end surfaces to one another, said disc having opposite lateral faces exposed in facing juxtaposition to substantially parallel exposed arc-supporting end surfaces of said first and second conductive electrodes to provide longitudinal gaps between the lateral faces of said disc and said first and second conductive electrodes, said electrodes, said disc and said spacers being peripherally joined to provide hermetically sealed chambers between said electrodes and said disc joined by the bore in said disc.
 4. The device according to claim 3 wherein said disc has a first thickness extending within said spacers providing said opposite faces in juxtaposition with said exposed arc-suppoRting end surfaces, said disc having a peripheral portion of reduced thickness between and joined to said spacers, with said disc just inwardly from said peripheral portion defining a shoulder adjacent joints formed between said peripheral portion and said spacers.
 5. The device according to claim 4 wherein said disc is further provided with an axial flange at the outer edge of said peripheral portion of reduced thickness, said axial flange extending adjacent and over a portion of at least one of said cylindrical insulating spacers.
 6. The device according to claim 3 wherein said central axial bore in said disc has a diameter between approximately one-third and three-fourths of the diameters of said end surfaces of said conductive electrodes.
 7. The device according to claim 3 wherein the second ends of said first and second conductive electrodes providing said arc-supporting end surfaces are substantially planar over the major portion thereof.
 8. The device according to claim 3 wherein the second ends of said first and second conductive electrodes providing said arc-supporting end surfaces are provided with a plurality of small cavities containing low work function material.
 9. The device according to claim 3 wherein the sum of the longitudinal distances between said arc-supporting end surfaces and the lateral faces of said disc along substantially any longitudinal line intersecting said arc-supporting end surfaces is less than the closest longitudinal distance between said arc-supporting end surfaces.
 10. The device according to claim 9 wherein the diameter of the bore in said disc is between approximately one-third and three-fourths of the diameter of the said arc-supporting end surfaces.
 11. The device according to claim 3 wherein the portions of said electrodes between the ends thereof are substantially cylindrical and closely adjacent the cylindrical interior walls of said spacers.
 12. The device according to claim 3 wherein said spacers are formed of ceramic material. 